Planned retrofit- Establish AT layer at existing external render layer- New windows and doors- EWI on top of external leaf- Fill cavities totally and seal top of cavity- (at this stage) use cold loft and have EWI meet loft roll

So with great trepidation here's my first detail. Almost as simple as it gets - the eaves with the original CW wrapped in EWI.

Note the ventilation gap at the top of the rafters. Currently there are no actual vents for the gap - the soffit board has no vents. Is the full-filling of the soffit with EWI therefore a problem?

It's usual to mark the airtightness barrier with a red line to indicate both its position and its continuity. Where does it go after the wall plate, for example? An average builder won't know what AT is. You will have to indicate exactly what the airtighness barrier actually is, and how it is joined together at junctions.

You seem to show 'loft roll' immediately below the tiles (with a gap). Is there no sarking or membrane? Most loft rolls I have seen do not like water. Similarly, what keeps the top of the EPS dry and prevents water settling on top of the cavity?

Nice, but FWIW, I suspect that the "roof tiles" really need to be shown being *overlapping* *over* the fascia board...In fact the latter could be raised, so that it cants the starter course upwards some...

(also it is not explicitly stated what the diagonal thin black line is...) (I imagine zinc / copper flashing or simlilar ?)and why not add tiles *to* the fascia board while U R at it...gg

Posted By: djhIt's usual to mark the airtightness barrier with a red line to indicate both its position and its continuity. Where does it go after the wall plate, for example? An average builder won't know what AT is. You will have to indicate exactly what the airtighness barrier actually is, and how it is joined together at junctions.

It actually is red! But sketchup adds the edge borders, I'll see if I can do something about that. Might need to use other software to add it.

After the wall plate - that's something I wanted to mention. It doesn't go anywhere! I am explicitly noting that's the extent of the AT layer. I am intending as a second phase of the retrofit to convert the roof to a warm roof, building the AT layer in there. I'm intending on an air test after this work to see how much of an issue lack of an AT layer across the ceiling is, and if necessary (i.e. the numbers look bad on PHPP) I'll fix it myself in the cold loft. Otherwise, wait for the warm roof.

What was your process for deciding this... look up the manufacturers and do what they say?

Posted By: djhYou seem to show 'loft roll' immediately below the tiles (with a gap). Is there no sarking or membrane? Most loft rolls I have seen do not like water. Similarly, what keeps the top of the EPS dry and prevents water settling on top of the cavity?

Sketchup again! There is a bitumen layer (1mm) under the battens.

- Add label for felt

EPS - yes, great. Now this was another concern. Obviously the felt will keep it dry, but belt and braces, I was told I had to have a good enough slope on the eaves insulation to make sure water didn't sit and freeze. Do you know what that angle is? Looks ok to me...

Posted By: gyrogearNice, but FWIW, I suspect that the "roof tiles" really need to be shown being *overlapping* *over* the fascia board...In fact the latter could be raised, so that it cants the starter course upwards some...

The first bit, yeah. The second bit about starter courses... is that important for a detail?

Do you not need battens and counter battens so the space between roof tiles and felt gets well vented and water does not pool along the batten? The felt is then laid flat (not saggy), and there is no issue with insulation of any kind touching it, or for ventilation of the roof space.

I must admit I have no forgotten exactly why we did it so there could be other reasons and benefits.

Posted By: GreenfishDo you not need battens and counter battens so the space between roof tiles and felt gets well vented and water does not pool along the batten? The felt is then laid flat (not saggy), and there is no issue with insulation of any kind touching it, or for ventilation of the roof space.

Interesting. Only one direction of batten at the moment. Does what you describe not just ventilate the battens and tiles though, given the felt is a bitumen? Isn't the purpose of ventilation here to also ventilate the roof structure (rafters etc)?

Is this a standard approach? I don't want to build something "proprietary" into the detail if it can be done another way (as I suggested above, batten the fascia board out slightly)?

Bugger, looks like I've only put in 20mm ventilation atm, AIUI should be 25mm (according to above link).

Is Tescon Vana suitable for plasterboard? It says non-mineral... not sure if plasterboard is considered mineral or not. According to application docs it can be applied to masonry with Orcon F, so have written that.

Posted By: gravelldAfter the wall plate - that's something I wanted to mention. It doesn't go anywhere! I am explicitly noting that's the extent of the AT layer. I am intending as a second phase of the retrofit to convert the roof to a warm roof, building the AT layer in there. I'm intending on an air test after this work to see how much of an issue lack of an AT layer across the ceiling is, and if necessary (i.e. the numbers look bad on PHPP) I'll fix it myself in the cold loft. Otherwise, wait for the warm roof.

You seem to have all the makings of a disaster planned in here. The usual plan is to complete the whole airtightness barrier and then *while it is still exposed* test it and repair any problems shown up. Not having a complete airtightness barrier makes testing difficult, and covering up parts of the airtightness barrier makes repairs difficult. The result can be a less than optimal result, with no sensible way to improve it.

Posted By: djhYou seem to have all the makings of a disaster planned in here. The usual plan is to complete the whole airtightness barrier and then *while it is still exposed* test it and repair any problems shown up. Not having a complete airtightness barrier makes testing difficult, and covering up parts of the airtightness barrier makes repairs difficult. The result can be a less than optimal result, with no sensible way to improve it.

Disaster in what sense? As in... a danger to the building type of disaster?

This is an incremental retrofit. There is not enough budget to complete the entire house.

In terms of testing the newly implemented AT layer, are there particular reasons why we can't test it while it is still exposed (i.e. in this case with the bottom courses of tiles out and wall parge coated where required)? Where do you consider it covered up?

Posted By: djhIs your roof membrane taut or draped?

Don't know what that means. As above it's old school bitumen felt, likely 1950s vintage, battens nailed through it onto the rafters.

Eps in eaves box follows slope of roof to top outside corner of wall plate, pir goes 200mm over top of eps with 25mm void above then steps void to 25mm plus thickness of pir, hopefully 75mm, above the wall plate use pir as a guard to stop quilt blocking ventilation space which will be 25mm to top of quilt

In terms of testing the newly implemented AT layer, are there particular reasons why we can't test it while it is still exposed

You can't test an airtight barrier except when the airtight barrier is complete. As I understand it, you are planning to complete the wall insulation before you start the ceiling and roof. So the wall barrier will be covered before you are in a position to test the whole.

IMHO, a better plan is to complete the entire air barrier first. Then test it and correct any deficiencies. Only then start to cover up the airtightness barrier.

Posted By: djhYou can't test an airtight barrier except when the airtight barrier is complete. As I understand it, you are planning to complete the wall insulation before you start the ceiling and roof. So the wall barrier will be covered before you are in a position to test the whole.

IMHO, a better plan is to complete the entire air barrier first. Then test it and correct any deficiencies. Only then start to cover up the airtightness barrier.

No, I wasn't hoping to complete the EWI before the eaves. I think the opposite; is there something in the detail which precludes this?

Rough order in my mind:

- Extend gable ends- CWI- Dig founds trench- Parge coat plinth- Knock out popped render on existing render and open existing cracks- Fill in the exposed holes- Parge coat top of wall in the eaves and treat as per the detail- Install windows (detail on its way!), tape to AT layer- Air test- EWI

Now, to semantics!

Surely an air barrier is never complete, truly. Any house can be air tested, and the extent of its 'air barrier' (even if it hasn't been designed and implemented as an air barrier) tested as to how well air permeability is reduced. Even the best air test result ever was >0. So, even if my designed and implemented air barrier does not include the ceiling and only extends to just inside the eaves, the air permeability can be measured.

Therefore, even though a complete "designed" air barrier hasn't been completed it can still be measured.

In PHPP, what I have assumed I can achieve is something like 7m3/m2*h for this phase. At this stage this fits into the performance target, with a view that when the warm roof goes on we'll get closer to something like 3.

I think I understand what you are saying: I am just grasping at a target with no assurance it can be achieved? I was wondering about air testing in the above order, and if it was above 7, seeing where that was coming from and trying to fix that myself (maybe including the loft work).

Given there's already 400mm of mineral wool throughout the loft, and some of the loft is inaccessible due to the placement of cold water tanks in corners etc (edit: I suppose we could get to it from outside with more scaffolding), do you think there's an expedient way of completing the air barrier across the ceiling?

Do you think it's worth measuring the house before starting (I know it will be bad!).

The nature of incremental retrofit is that you cannot hold contractors to an AT target for anything but the final step (if the air barrier isn't complete).

Posted By: gravelldeven if mydesigned and implementedair barrier does not include the ceiling and only extends to just inside the eaves, the air permeability can be measured

Yes, you can measure it and no, the results won't be useful, IMHO.

Surely an air barrier is never complete, truly.

Rather the reverse. An air barrier is always complete - it's simply a surface enclosing a volume. But unless it is of similar 'quality' throughout, the results are going to be dominated by the 'worst' part. In order to get a meaningful idea of whether 'good' parts are performing as intended, it all needs to be to a comparable standard.

Do you think it's worth measuring the house before starting (I know it will be bad!).

Yes.

The nature of incremental retrofit is that you cannot hold contractors to an AT target for anything but the final step (if the air barrier isn't complete).

In my experience you cannot hold contractors to an airtighness target at all. The best you can do is encourage them. And a target doesn't make sense unless the airtightness barrier is complete. The way to meet a target is by quality control - both during construction and by testing and repair of defects.

Rather the reverse. An air barrier is always complete - it's simply a surface enclosing a volume. But unless it is of similar 'quality' throughout, the results are going to be dominated by the 'worst' part. In order to get a meaningful idea of whether 'good' parts are performing as intended, it all needs to be to a comparable standard.

That's what I've always assumed, but never really understood to what levels of magnitude we are talking about "dominat[ing]".

What I assumed we could do was: run the air test, check the leakage paths and ascertain where the weakness is. But I guess: working out where the leakage is is actually non trivial, because air can come through any crazy path through the house.

Posted By: djh

Do you think it's worth measuring the house before starting (I know it will be bad!).

Yes.

Why? Especially given you said you thought the measurements wouldn't be useful from testing the part-barrier as I've suggested?

No doubt it is necessary to distinguish between air barriers in new builds, and air barriers in renovations - the latter poses particular problems and no doubt requires a different methodology.

In a new build, the AB can be implemented according to theoretical concepts, whereas in a renovation it needs to be implemented according to ad hoc situations, which in turn require innovative (?) (and perhaps less-than-perfect) solutions.

In which case it might be worthwhile devoting effort into identifying said retrofit approaches/solutions, and accepting that the finished result "might be somewhat different".

One approach that might be worthy of investigation, being how to optimise isolation of a particular volume in order to allow testing...

etc.

gg

(back to the bottle)

P.S.If the building were a boat (in French, the same word, BTW...), then one would fix the big leaks first, to keep it afloat, then fix the small leaks later, by selecting an appropriate sized plug...